The invention relates to liners for shaped charge devices used to perforate oil well casings and well bore holes. More particularly, the invention relates to an explosive jet liner which perforates an oil well casing without leaving a slug of metal in the resultant hole.
Shaped charges, capable of producing an explosive jet, have been used for many years to perforate oil well bore hole casings. In general, the charges are characterized by a shaped explosive charge housed in a container having one open end. The explosive has a concave surface facing the open end of the container aligned at the point the well casing is to be perforated. The concave surface is lined with a metallic liner to seal off the open end of the charge container. A compressive shock wave generated by detonation of the explosive charge collapses the liner. The inner portion of the liner is extruded into a narrow diameter high speed jet. The jet reaches a speed of about 10,000 m/sec. The remainder of the liner forms a larger diameter slug or "carrot". The slug is slower moving, traveling on the order of about 1000 m/sec and generally follows the path of the jet.
The well casing is perforated at depths where oil bearing earth formations are believed present. Oil flows into the well casing through the perforation holes. The slug has a tendency to embed in the perforated hole impeding the flow of oil into the well casing. The slug may also cause mechanical interlocking between the detonation tube holder which positions the shaped charge and the well casing. Much effort has been exerted to minimize or eliminate the slug.
U.S. Pat. No. 3,077,834 to Caldwell discloses minimizing the slug by forming the liner from loosely packed copper spheres. The spheres may be coated with a low melting metal such as tin to improve adhesion. The slug formed from compacted spheres is porous and fragile. When it strikes the wall of the well casing, the slug pulverizes and does not obstruct the flow of oil. Compacted powder liners now comprise about 90% of the oil well market. The liners are usually a mixture of copper and lead spheres containing about 20% by weight lead.
Compacted powder liners are not ideal. As disclosed in U.S. Pat. No. 3,196,792 to Charrin, cold pressed and/or sintered liners are not watertight. The bore hole is frequently filled with fluid. The liner may leak causing the explosive mixture to get wet and fail to detonate. The cold pressed, unsintered powder liners are fragile and prone to break during handling or assembly. The pressed powder surface has a large surface area producing liners which are hydroscopic. The moisture reduces the effectiveness of the explosive mixture. Compacted liners are formed individually increasing the cost. The uniformity of powder composition and compaction pressure may vary from liner to liner and from region to region within a liner. This variation leads to unpredictable jet performance.
The remaining 10% of the oil well market is comprised of wrought metal liners. Wrought metal liners do not have the problems associated with compacted powder liners. However, wrought liners formed from ductile metals and alloys can form relatively large slugs.
One solution has been bimetallic liners. U.S. Pat. No. 3,025,794 to Lebourg et al discloses a bimetallic liner comprised of a layer of copper and a layer of zinc. The ductile copper forms the perforating jet. The zinc vaporizes as the slug is accelerated eliminating the slug. Bimetallic liners have the disadvantage that two layers are bonded together. The quality of the bond influences the jet performance. The extra forming steps add to the cost of the liner.
Wrought metal liners formed from specific alloys have also been disclosed to minimize slug formation. U.S. Pat. No. 3,128,701 to Rinehart et al discloses liners which melt at temperatures of less than 500.degree. C. Among the alloys and metals disclosed are 50% lead/50% tin, 97.6% zinc/1.6% lead and lead, zinc or cadmium metal. The liners melt as the slug travels to the well casing. The molten slug does not obstruct the perforated hole.
U.S. Pat. No. 3,112,700 to Gehring, Jr. discloses binary eutectic alloy liners. The slug is minimized by forming a highly ductile metal matrix with brittle dendrites, uniformly, but discontinuously dispersed throughout the matrix. Among the eutectic compositions disclosed are 88.8% Pb/11.2% Sb, 61.9% Sn/38.1% Pd and 71.9% Ag/28.1% Cu. While these alloys may reduce slug formation, they are not as easily shaped as more ductile metals such as copper and copper alloys.
U.S. Pat. No. 4,958,569 discloses a wrought metal liner for a shaped charge device having a ductile metal matrix and discrete second phase. The alloy composition is selected so that the second phase is molten when the liner is accelerated following detonation. The molten phase reduces the tensile strength of the matrix so the liner slug is pulverized on impact with a well casing. The slug does not penetrate the hole perforated in the well casing by the liner jet and oil flow is not impeded. Alternatively, the second phase forms discrete crack nucleation sites which result in the slug pulverizing on impact.
The present invention is based on the discovery of a group of castable copper alloys offering advantages similar to those realized from the use of wrought copper alloys as taught in U.S. Pat. No. 4,958,569. In fact, due to the nature of the casting process, certain advantages are realized. For example, a higher concentration of additions may be made to the base alloy. The size of the second phase may be adjusted by control over the cooling rate.
Accordingly, it is an object of the invention to provide a copper alloy which is cast directly to a shape suitable for use in shaped charge liners. It is a feature of the invention that the copper alloy is multiple phase having a ductile matrix and a discrete dispersion of a second phase. It is a further feature of the invention that the second phase may either melt upon detonation of the shaped charge device or form brittle crack nucleation sites. It is an advantage of the invention that by direct casting the shaped charge liners, unique microstructures may be obtained.
Yet another advantage of the invention is that the volume concentration of the second phase may be increased beyond that feasible for wrought alloys. Still another advantage of the invention is that the second phase may be either homogeneous throughout the liner or non-homogeneous. Another advantage of the invention is that additional elements may be added to induce planes of weakness. Still another advantage is the copper alloy liners may be formed by conventional casting or thixoforging.
In accordance with the invention, there is provided a metal liner for a shaped charge device of the type formed from a multiple phase alloy. The liner has a ductile metal matrix and a discrete second phase dispersed within that matrix. The second phase is selected to have a melting temperature less than the temperature reached by the metal liner following detonation. The multiple phase alloy is cast directly to a shape effective to compress on detonation of the shaped charge device.
The above stated objects, features and advantages will become more apparent from the specification and drawings which follow.